1. Field of the Invention
The present invention relates to a brushless motor drive device, and, in particular, to a drive device which sensorlessly determines energization mode switching of a three-phase brushless motor.
2. Description of Related Art
Japanese Laid-open (Kokai) Patent Application Publication No. 2009-189176 discloses a drive system of a synchronous motor that: in a three-phase synchronous motor, detects an induced voltage (pulse induced voltage) of a non-energized phase induced by a pulse voltage; compares the level of this induced voltage with that of a reference voltage; and sequentially switches energization modes according to the result of this level comparison.
Incidentally, a pulse induced voltage of a non-energized phase is detected while a pulse-shaped voltage is being applied to two phases. However, since the pulse induced voltage oscillates immediately after the start of a voltage application, if the duty ratio of the pulse voltage is small (if the voltage application time is short), there is a possibility that the pulse induced voltage may be sampled within the oscillation period, and consequently the pulse induced voltage may be detected falsely, and the timing of switching energization modes may be misjudged.
Moreover, the magnitude of the pulse induced voltage of the non-energized phase changes according to the duty ratio of the pulse voltage, and if the duty ratio is small, the pulse induced voltage becomes below the reference voltage, resulting in a possibility that determination of the switching timing of energization modes may become impossible.